The versions of Remmina and the FreeRDP libraries that it uses date back to Ubuntu 14.04, are now slightly outdated. Fortunately, there is a PPA for the latest Remmina, which makes updating easy:
sudo apt-add-repository ppa:remmina-ppa-team/remmina-next sudo apt-get update sudo apt-get install libfreerdp-plugins-standard remmina remmina-plugin-rdp
After the update, make sure that you have completely closed all current copies of Remmina by selecting "Exit" from the tray icon, and then open it again.
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